Google not looking at content

Google not looking at content or their algorithm is not what it says it is in my opinion.

I have noticed on one of my websites the content to google does not matter. I have a pet transport website and all of a sudden started to slide down from the first place down to about 65th or so in a matter of a day or two.
This website is all about dogs, cats and pet transport and associated content. Like pet passport, pet travel etc. I am also updating the main page once per month.
Took me about 2 months to bring it back to the first page first and second position. I have not used paid advertising only content and SEO.
Than all of a sudden today in a matter of hours I was back down to the 40th position for the main keyword and anywhere from the 15th up to 57th for most of the others.

The websites that is up front now are pet food, parcel delivery companies, some other transporters with one page websites, pet stores, bus company, and alike.
Yoast premium tells me my SEO and content is right on the spot.

So, that leaves nobody else but Google screwing something up or their algorithm is not working with content, but with related associated content.